The high electron mobility transistor is an important element in the field of high power and high frequency applications. As shown in FIG. 1, a high electron mobility transistor 1 using AlGaN/GaN as a double epitaxial layer according to the prior art comprises an aluminum nitride (AlN) layer 12, and an aluminum gallium nitride (AlxGa1-xN) layer 13 formed above the aluminum nitride layer 12. The aluminum nitride layer 12 is formed on a substrate 11, for example, a silicon (Si) substrate, and the aluminum nitride layer 12 is used to alleviate the problem of lattice mismatch between the substrate 11 and the aluminum gallium nitride layer 13. The aluminum content x of the aluminum gallium nitride (AlxGa1-xN) layer 13 is ranged from 1 to 0 according to the prior art, and there are a gallium nitride buffer layer 14, a gallium nitride channel layer 15, a barrier layer 16 and a gallium nitride covering layer 17 sequentially formed above the aluminum gallium nitride layer 13. There are a source 171, a drain 173 and a gate 172 individually formed on the gallium nitride covering layer 17.
However, a thickness of the aluminum nitride layer 12 of the high electron mobility transistor 1 according to the prior art is usually less than 100 nm, so that it cannot compensate the lattice mismatch between the substrate 11 and the aluminum gallium nitride layer 13 due to the thickness is too thin. Thus, a high dislocation can be generated easily, such that cracks occur when a thickness of the gallium nitride (GaN) grown on the substrate 11 is over 1 μm. Also, the defect density would exceed 109 cm−2, which results in reducing impedance of the epitaxial film, and the crystallization quality cannot be effectively improved. In other words, the high electron mobility transistor 1 of the prior art still has the problem of a large amounts of the vertical leakage current, resulting in lowering the breakdown voltage.
Consequently, how to provide a high electron mobility transistor with reduced vertical leakage current and improved breakdown voltage is one of the issues desired to be solved in the art.